1. Field of the Invention
This invention relates generally to a radio receiver which has the ability to select any one of a plurality of incoming frequencies whose amplitude is above a certain predetermined threshold level. In particular, the invention is directed to a tour guide system including an automatic frequency selecting means to help overcome the problem of background noise and cross-talk suppression.
2. Description of the Prior Art
A description of prior art tour guide systems may be found in my copending application entitled: "A Tour Guide System Including Means For Noise And Cross Talk Suppression", Ser. No. 454,519, Filed Mar. 25, 1974 now U.S. Pat. No. 3,934,202.
In a typical tour guide system, antenna loops are placed in the ground or floor of the tour area in a side-by-side relationship. The visitor on the tour carries with him a radio receiver which picks up the signal generated in that particular loop. As the visitor moves from loop to loop he receives a different program.
One of the difficulties with the use of a plurality of closed loops is that there is apt to be a great amount of cross-talk between loops. Therefore, a visitor listening to the message of one loop may, inadvertently, hear part of the message being delivered in one or more adjacent loops. In order to minimize the problem of cross-talk between loops in tour guide systems, several alternatives are possible. One technique is to use FM communications but, unfortunately, this leads to relatively complex and sophisticated circuitry which, in turn, produces higher costs. Such an approach is discussed in a patent to Abel, U.S. Pat. No. 3,474,451. Another technique, which was first employed by this inventor in the context of a tour guide system is to use a compressor in the transmitter and an expander in the receiver. While this technique does work well, it is still relatively expensive.
Another approach to minimizing cross-talk is to broadcast loop messages at different loop frequencies. This approach significantly reduces the problems of noise and cross-talk between loops, but then raises the necessity of having to change frequencies when going from loop to loop. If this is done manually it can be annoying and inconvenient to the tour guest. Therefore, a means was sought which would automatically select the correct frequency as the visitor goes from loop to loop. While automatic searching devices are known in the prior art, this is believed to be the first time that this technique has been used in the context of tour guide systems. More importantly, this is believed to be the first time that this particular automatic frequency selecting means has been used to select frequencies for a radio receiver or the like.
One advantage of the instant invention is that a superheterodyne type radio receiver is not necessary since a simpler, linear receiver will do just as well at a much lower cost. In was in the context of the foregoing prior art that the following invention was perfected.